Substrates such as wafers are manufactured by highly complicated manufacturing processes. These manufacturing processes should be monitored in order to ensure the quality of the wafers.
Some substrates include holes of high aspect ratio. An aspect ratio is the ratio between a depth of a hole and a width of hole.
A high aspect ratio hole can be defined as a hole that has an aspect ratio that is high enough so that when illuminating the bottom of the hole the electrons that are emitted from the bottom of the hole do not exit the hole.
When illuminating the bottom of a high aspect ratio hole the bottom seems dark—regardless of the composition of the bottom of the hole.